#oxidative stress

Cell Organelles

The major function of peroxisomes is:

Peroxisomes are single membrane oxidative organelles discovered by Rhodin and De Duve, numbering hundreds per cell, containing more than fifty enzymes producing and degrading hydrogen peroxide. Beta-oxidation of very long chain fatty acids greater than twenty two carbons, branched phytanic acid alpha-oxidation, bile acid intermediate shortening and ether lipid synthesis of plasmalogens providing antioxidant membranes and myelin components occur via enzymes acyl-CoA oxidase generating H2O2, bifunctional enzyme and thiolase. Catalase converts two H2O2 to water and oxygen detoxifying, also urate oxidase and D-amino acid oxidase produce peroxide. Import depends on peroxisomal targeting signals PTS1 tripeptide SKL at C terminus recognized by Pex5 receptor and PTS2 nonapeptide recognized by Pex7. Protein degradation via lysosomal hydrolases acidic, ATP synthesis mitochondrial inner membrane, nucleic acid processing nuclear. Defects in Pex genes cause Zellweger spectrum with accumulation of very long chain fatty acids, hypotonia and neuronal migration failure illustrating essential lipid metabolic and detoxification function distinct from energy metabolism.

Ref: Wanders Annu Rev Biochem; peroxisome beta-oxidation VLCFA catalase PTS1 PTS2 Pex import.

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Microbial Physiology

What is the role of superoxide dismutase in aerobic bacteria?

Superoxide radical generated when flavoenzymes and respiratory chain components univalently reduce oxygen, highly toxic inactivating dehydratases bearing exposed iron sulfur clusters like aconitase and fumarase releasing free iron that fuels Fenton chemistry producing hydroxyl radical. Superoxide dismutase constitutes primary defense dismutating two superoxides into dioxygen and hydrogen peroxide reaction two O2 superoxide plus two protons to O2 plus H2O2. Enzymes classified by metal cofactor manganese SodA inducible by SoxRS, iron SodB constitutive, copper zinc SodC periplasmic exported via Sec pathway. Periplasmic SodC crucial for pathogenic survival inside macrophages generating oxidative burst. Product H2O2 subsequently detoxified by catalase KatG KatE and alkyl hydroperoxide reductase AhpCF. Without SOD cells show auxotrophy for branched chain amino acids, elevated mutation frequency, and sensitivity to redox cycling agents paraquat. Therefore enzyme role is conversion of superoxide into oxygen and peroxide, not DNA repair nor ATP enhancement, intermediate step in reactive oxygen species detoxification cascade protecting metalloproteins and genome integrity during aerobic growth.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 6: Superoxide dismutase - ROS detoxification.

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Microbial Physiology

Reactive oxygen species (ROS) are detoxified by:

Aerobic respiration inherently leaks electrons to oxygen at Complex I and Complex II producing superoxide anion which dismutates to hydrogen peroxide and can generate highly destructive hydroxyl radical via Haber-Weiss and Fenton chemistry catalyzed by iron. These reactive oxygen species damage virtually all macromolecules including proteins forming carbonyls, lipids forming peroxides and DNA forming 8-oxoguanine leading to mutations. Bacteria deploy multilayered detoxification network. Superoxide dismutase encoded by sodA manganese cofactored, sodB iron cofactored and sodC copper-zinc periplasmic catalyzes rapid dismutation of superoxide to peroxide and oxygen lowering steady state to picomolar protecting dehydratases. Catalase isozymes KatG bifunctional and KatE monofunctional then convert peroxide to water and oxygen, while alkyl hydroperoxide reductase AhpCF scavenges low-level organic peroxides. Coordinated transcriptional induction via OxyR sensing peroxide and SoxR sensing superoxide ensures prompt response proportional to stress. Peptidoglycan hydrolase remodels wall, DNA helicase unwinds duplex, transpeptidase cross-links wall unrelated to ROS, distinguishing catalase and superoxide dismutase as canonical evolutionarily conserved ROS detoxifiers essential for aerobic growth and pathogenesis inside phagocytes.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Catalase and Superoxide Dismutase in ROS Detoxification.

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Microbial Physiology

Which bacterial enzyme neutralizes hydrogen peroxide?

Hydrogen peroxide is unavoidable byproduct of flavoprotein oxidases such as NADH oxidase and autoxidation of menaquinones generating reactive oxygen stress capable of hydroxyl radical formation via Fenton reaction damaging iron-sulfur clusters and DNA. Aerobic bacteria depend on extremely efficient enzymatic detoxification. Catalase is tetrameric heme enzyme containing heme b active site where first H2O2 oxidizes ferric heme to Compound I ferryl oxo porphyrin radical, second H2O2 reduces Compound I releasing water and dioxygen. Catalytic rate is among highest known about ten million molecules per second per active site, essentially diffusion limited. Genes encoding catalases katG catalase-peroxidase and katE are induced via OxyR peroxide sensor recognizing hydrogen peroxide and general stress sigmaS RpoS during stationary phase. DNA polymerase replicates chromosome, RNA helicase unwinds RNA secondary structures, topoisomerase decatenates chromosomes, none scavenge peroxide, leaving catalase together with peroxidases like alkyl hydroperoxide reductase AhpCF as primary peroxide defense system distinguishing them from central information processing enzymes and explaining positive bubble test for catalase-positive organisms.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Catalase Function and Hydrogen Peroxide Neutralization.

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Microbial Physiology

Which molecule acts as an electron donor in bacterial oxidative stress response?

Redox homeostasis during oxidative stress requires continuous electron flow from reduced pyridine nucleotide NADPH generated by pentose phosphate pathway to thiol antioxidant enzymes. Thioredoxin is small 12 kDa dithiol protein with highly conserved WCGPC active motif forming reversible intramolecular disulfide acting as mobile electron shuttle. Reduced thioredoxin provides low-potential electrons directly to peroxiredoxins such as AhpC and Tpx that reduce hydrogen peroxide and organic peroxides to water and alcohols, to methionine sulfoxide reductases MsrA and MsrB that repair oxidized methionine residues in damaged proteins, and to ribonucleotide reductase converting ribonucleotides to deoxyribonucleotides for DNA synthesis. It also reduces OxyR disulfide when peroxide stress subsides and repairs aberrant disulfides formed in cytoplasmic proteins. Peroxiredoxin is acceptor not donor, peptidoglycan is envelope heteropolymer, DNA gyrase is topoisomerase modulating supercoiling, so thioredoxin uniquely functions as principal electron donor linking NADPH to diverse detoxification and biosynthetic reactions, explaining its essentiality and deep conservation across life.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 7: Thioredoxin as Electron Donor in Oxidative Defense.

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Microbial Physiology

Which enzyme neutralizes hydrogen peroxide (H₂O₂) in bacterial cells?

Hydrogen peroxide at even low micromolar concentrations of 1 to 2 micromolar causes inactivation of mononuclear iron enzymes and iron-sulfur dehydratases such as fumarase via Fenton chemistry generating highly reactive hydroxyl radicals that oxidize DNA generating 8-oxoguanine and protein carbonyls leading to mutagenesis. Bacteria deploy specialized scavenging enzymes for peroxide detoxification. Catalases are predominantly heme-containing tetrameric enzymes containing protoheme IX or heme b in each active site that dismutate two molecules of H2O2 into water and molecular oxygen with extremely high turnover numbers approaching diffusion-limited rates around 10^6 to 10^7 per second and Michaelis constants in millimolar range. Monofunctional catalases KatG possessing catalase-peroxidase bifunctionality and KatE plus manganese catalases are induced by OxyR, PerR and during stationary phase under general stress sigma factor RpoS. Catalase activity is readily assayed macroscopically by vigorous bubble formation when colony is exposed to 3 percent peroxide solution. DNA helicase unwinds DNA duplex for replication, RNA polymerase alpha-beta complex transcribes genes to mRNA, duplication in provided options likely reflects typographic error, but catalytic degradation of peroxide remains exclusive biochemical function of catalase, often operating redundantly with alkyl hydroperoxide reductase AhpCF and glutathione peroxidases in multilayer antioxidant defense protecting genome integrity.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Catalase and Hydrogen Peroxide Detoxification.

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Microbial Physiology

The primary function of glutathione in bacterial stress response is:

Glutathione is a low-molecular-weight tripeptide gamma-glutamyl-cysteinyl-glycine present at millimolar concentrations up to 10 mM in many Gram-negative bacteria such as Escherichia coli and few Gram-positives that synthesize it via GshA and GshB. Reduced form GSH serves as major thiol buffer and electron donor protecting against reactive oxygen species generated continuously by endogenous respiratory electron transport and by host immune oxidative burst involving NADPH oxidase-dependent production of superoxide during phagocytosis. It directly scavenges superoxide anion, hydroxyl radical and hydrogen and organic peroxides via glutathione peroxidases, forming oxidized glutathione disulfide GSSG that is rapidly recycled by NADPH-dependent glutathione reductase Gor maintaining high GSH to GSSG ratio of greater than 100 to 1. GSH also forms mixed disulfides with redox-sensitive cysteine residues in proteins via reversible S-glutathionylation, temporarily shielding them from irreversible overoxidation to sulfinic or sulfonic acids that require repair. It acts as cofactor for glutathione peroxidases, glyoxalases that detoxify methylglyoxal, and for detoxification of electrophiles and xenobiotics via glutathione S-transferases. Depletion via mutation in gshA renders cells hypersensitive to oxidants, linking its primary role to redox homeostasis rather than to DNA replication or flagellar motility.

Ref: Prescott's Microbiology, 11th ed., Chapter 7: Glutathione and Oxidative Stress Protection in Bacteria.

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Microbial Physiology

The Thioredoxin system in bacteria is essential for:

Maintenance of intracellular reducing environment and proper thiol-disulfide balance is crucial for protein function, enzyme activity and protection against oxidative damage that would otherwise cause aggregation. The thioredoxin system consists of small 12 kDa redox-active protein thioredoxin TrxA with highly conserved active site motif WCGPC containing two cysteines, flavoprotein thioredoxin reductase TrxB that transfers electrons from NADPH to oxidized thioredoxin via FAD and target disulfide-containing proteins. During oxidative stress or normal oxidative protein maturation in periplasm, inappropriate intermolecular and intramolecular disulfide bonds form in cytoplasmic proteins. Reduced thioredoxin with dithiol performs nucleophilic attack on aberrant disulfide, forming transient mixed disulfide intermediate then resolving through second cysteine to restore native reduced thiols in target and become oxidized itself. TrxB then re-reduces TrxA using NADPH as electron source, completing cycle. This redox cycle also supplies essential electrons to class I ribonucleotide reductase NrdAB for deoxyribonucleotide synthesis required for DNA precursor supply, to methionine sulfoxide reductases MsrA/MsrB that repair oxidized methionine residues, and to peroxiredoxins that detoxify peroxides, explaining essentiality beyond direct antioxidant defense and distinction from activities that inhibit replication or translation.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 21: Thioredoxin System and Disulfide Bond Reduction.

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Microbial Physiology

PerR, OxyR, and SoxR regulate bacterial responses to:

Aerobic metabolism inevitably generates reactive oxygen species including superoxide radical anion and hydrogen peroxide via single-electron leakage from respiratory flavoproteins and autoxidation of reduced ferredoxins that damage solvent-exposed iron-sulfur clusters, DNA bases and unsaturated lipids. Bacteria have evolved exquisite sensing transcription factors that regulate defensive regulons. OxyR in Escherichia coli contains highly reactive cysteines C199 and C208 that when oxidized by low micromolar H2O2 form a reversible intramolecular disulfide bond driving conformational change to active tetramer that binds promoters and activates transcription of katG catalase-hydroperoxidase, ahpCF alkyl hydroperoxide reductase, dps ferritin-like DNA-binding protein that sequesters iron, and gorA glutathione reductase. PerR in Gram-positive Bacillus subtilis senses peroxide through metal-catalyzed histidine oxidation at the regulatory site where bound Fe2+ or Mn2+ mediates Fenton-mediated oxidation, causing derepression of regulon including catalase KatA and peroxiredoxins AhpC. SoxR contains a solvent-exposed [2Fe-2S] cluster oxidized directly by superoxide and redox-cycling agents like paraquat, inducing transcription of soxS regulator which in turn activates sodA manganese superoxide dismutase, fumC fumarase C resistant to ROS, and acrAB tolC efflux pump. Together these three factors orchestrate metal sequestration, DNA protection and enzymatic detoxification.

Ref: Storz and Imlay, Oxidative Stress Regulators, Annu Rev Microbiol: OxyR, PerR, SoxR Oxidative Stress Systems.

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